A varistor has the property that the resistance is high at low voltage but low at high voltage. A varistor comprises a varistor body, which is usually cylindrical, with two parallel end faces. The end faces are provided with electrodes for contacting and current distribution. These electrodes are in the form of a layer of electrode material. The layer may consist of aluminium or zinc or another metal. The layer may also consist of a conductive ceramic, as is clear from WO 8910813.
To carry large currents through the varistor body, the current has to be distributed as uniformly as possible over the end faces. For this, the properties of the layer play a major roll.
The varistor body is formed, for example, by pressing metal oxide powder, whereupon the pressed body is sintered, preferably in the temperature interval of 1100-1300° C. for about 2-10 h.
After the sintering, the end faces of the varistor body are usually ground or lapped.
After the grinding, the end faces of the varistor body are coated with a layer of electrode material. The detailed shape of the layer is determined by the risk of flashover or damage due to skin effect.
Layers of electrode material are usually applied to the end faces of the varistor bodies by metallizing, preferably by arc spraying or flame spraying of aluminium or zinc. The thickness of the layer is usually about 50 micrometers. Layers of electrode material, which have been applied according to the above-mentioned methods, are characterized by inhomogeneities, thickness variations, a relatively high contact resistance, a high surface roughness, difficulties with the corrosion resistance, and internal stresses at the boundary layer.
It is known that relatively thin layers of gold, on experimental samples of polymer material containing ZnO as filler, have been coated by sputtering (see AC Conductivity Effects of Non-linear Fillers in Electrical Insulation, 2000 Conference on Electrical Insulation and Dielectric Phenomena, p. 133. This method, which is not related to a polymer material with a filler, has not yet been commercialized.
GB 1508327 describes a varistor with several input connections, the purpose of which is to provide protection against voltage transients in polyphase circuits. The cylindrical varistor body contains diametrical sections in one end face, forming “segments” of varistors which are contacted by electrodes applied, for example, by means of sputtering. One disadvantage of such a varistor is that is has limited current and energy absorption capability.
The capacity to withstand repeated electrical loads, for example impulse currents for periods of about 4-20 μs, without breaking down is referred to as the high-current capability. This is described, for example, in the U.S. Pat. No. 6,199,268 B1.
This skin effect adjacent to the periphery of the layer may lead to local overheating of said varistor, and hence to failure, by electrothermal instability. The capacity to withstand high impulse currents for periods of the order of magnitude of 0.5 ms or longer without breaking down is referred to as the energy absorption capability.